Plasma arc torches are widely used in the cutting of metallic materials. A plasma arc torch generally includes a torch body, an electrode mounted within the body, a nozzle with a central exit orifice, electrical connections, passages for cooling and arc control fluids, a swirl ring to control the fluid flow patterns, and a power supply. The torch produces a plasma arc, which is a constricted ionized jet of a plasma gas with high temperature and high momentum. Gases used in the torch can be non-reactive (e.g. argon or nitrogen), or reactive (e.g. oxygen or air).
In operation, a pilot arc is first generated between the electrode (cathode) and the nozzle (anode). The pilot arc ionizes gas passing through the nozzle exit orifice. After the ionized gas reduces the electrical resistance between the electrode and the workpiece, the arc transfers from the nozzle to the workpiece. The torch may be operated in this transferred plasma arc mode, which is characterized by the conductive flow of ionized gas from the electrode to the workpiece, for the cutting of the workpiece.
Contact starting is one known technique for generating the pilot plasma arc. Contact starting is advantageous because it does not require high frequency equipment and does not generate electromagnetic interference. In one form of contact starting, the electrode is manually placed into electrical connection with the workpiece. A current is then passed from the electrode to the workpiece and the arc is struck by manually backing the electrode away from the workpiece.
Improvements in plasma arc torch systems have been developed which have eliminated the need to strike the torch against the workpiece in order to initiate an arc, thereby avoiding damage to brittle torch components. One such system is disclosed in U.S. Pat. No. 4,791,268 ("the '268 patent"), which is assigned to the same assignee as the instant application. The '268 patent describes a torch having a movable electrode and a stationary nozzle. A spring coupled to the electrode causes it to initially contact the nozzle such that the nozzle orifice is blocked. To start the torch, current is passed through the electrode and nozzle while a plasma gas is supplied to a plasma chamber defined by the electrode, the nozzle, and the swirl ring. Contact starting is achieved when the buildup of gas pressure in the plasma chamber overcomes the spring force, separating the electrode from the nozzle and drawing a low energy pilot arc therebetween. Thereafter, by bringing the nozzle into close proximity with the workpiece, the arc may be transferred to the workpiece with control circuitry increasing electrical parameters to provide sufficient energy for processing the workpiece. Plasma arc torch systems manufactured according to this design have enjoyed widespread acceptance in commercial and industrial applications.